1. Field of the Invention
The present invention relates to an extrusion head having upper and lower clamping elements which are movable against and act on a die plate (also called a final die, finishing die or end plate and hereafter also referred to as "end plate") for clamping the die plate and a plate holder (also called preforming die, plate holder or cassette and hereafter referred to as "cassette") to the extrusion head.
2. Discussion of the Prior Art
Extrusion tools which process rubber mixtures are provided with extrusion heads that contain clamping devices which comprise the clamping elements proper and drive units for moving the clamping elements. The drive units utilized in the clamping devices are primarily hydraulic cylinders. The clamping elements may be configured as clamping wedges as disclosed, for example, in GDR (German Democratic Republic) Pat. No. 124,369 to which corresponds U.S. Pat. No. 4,137,027 or as pivotal levers as disclosed, for example, in German Offenlegungsschrift No. 2,457,532 to which corresponds United Kingdom Pat. No. 1,521,756.
At the end of the extension head is a cassette which has a recess that contains a final die or end plate. When the extrusion head is ready for operation, the clamping elements are engaged and forced onto the end plate with a force sufficient to sealingly press the end plate and the cassette against the extrusion head.
When a strip of a different profile is to be extruded, it is frequently necessary only to replace the particularly configured end plate, while the cassette may remain in place at the extrusion head. In practice, however, generally if the end plate is exchanged, the cassette must also be removed. This is so because the rubber material to be processed is able to flow between the head components of the extrusion head (i.e. the opposing faces of the cassette and the extrusion head) due to the fact that the external contact pressure on the cassette has been reduced following the retraction of the clamping wedges. When the extrusion head resumes operation, the rubber film between the head components causes the position of the components to deviate from the predetermined position and may also cause the clamping wedges to become wedged in their guides. It is a further disadvantage of having to replace the cassette together with the end plate that excessively long down times result for the entire extrusion system.
It is also known to eject the cassette from the extrusion head by the use of separate, laterally disposed cylinders.